It is known, particularly for vehicles using alternative fuels such as CNG, LNG and hydrogen, to mount a plurality of fuel cylinders to the roof of a vehicle. Typically, metal frames are used to secure the cylinders to the vehicle's roof, using metal structural members to absorb and transfer any impact away from the cylinders and the associated valving. Unfortunately, the addition of such apparatus adds considerably to the overall weight of the roof mounting system making these systems less than desirable. Further, damage to the structural members used to absorb and transfer the impact may ultimately result in loss of integrity of the cylinders should the members be sheared from the overall mounting system or be deflected significantly towards the cylinders in crash or high-impact situations.
In order to meet NFPA 52 2002 and CSA B109-01 certification in both the US and Canada, respectively, the mounting systems must be designed to withstand dynamic loading as a result of normal operation and in the event of a crash. Pressure vessels or cylinders must be restrained due to inertial loading as well as be protected from direct impact. The dynamic loads which must be safely restrained in the event of a crash are specified in terms of multiples of gravity. Frames are provided to absorb and meet requirements of the vehicle and further to surround the cylinders. Impact may also be directed to the roof mounting itself. The loading designs must accommodate both longitudinal and transverse orientation of the vessels and is dependant upon the standards to be met. In Canada, cylinders are typically oriented in the same direction as travel of the vehicle. Dynamic loading must be at least 20 g in the longitudinal direction of the vehicle and 8 g in any other direction. Impact standards require protection of the fuel cylinders under specified mass and momentum conditions. These loads supersede those required for normal operation and are generally more stringent than those imposed in the US and in other pats of the world.
Frames are known which are resistant to inertial loading as a result of vehicle impact. In 1998, Lincoln Composites (Lincoln, Nebr., USA), a division of Advanced Technical Products, Inc., disclosed a modular concept for roof mounting utilizing a lightweight truss frame, expandable to accommodate various lengths of cylinders. Integration of the modules to the bus roof is accomplished by utilizing mounting brackets that can be relocated along the length of the modules to correspond with the roof “hard points”. The modular frame comprises end frames spacing two rails and a plurality of truss-like central frame members running lengthwise in the same direction as the cylinders and separating the cylinders, thus adding structural rigidity to the frame.
Other frames have been designed to meet safety requirements and weight restrictions. One such known design is that used typically for roof-mounting in low floor buses comprising a frame structure of end members and cross members. The frame has steel straps at two places along each pressure vessel, clamping each into the frame.
In the Lincoln Composites system described above, cylinders are positioned with their longitudinal axis oriented in the same direction as the longitudinal axis of the vehicle. In other known frames, cylinders are oriented with their longitudinal axis at 90 degrees to the frame rails and to the longitudinal axis of the vehicle. The differences in orientation of the cylinders are representative of differences in mounting conventions between North America and those in Japan and Europe.
U.S. Pat. No. 6,257,360 to Wozniak et al. teaches a plurality of compressed gas cylinders nested within shock-absorbing foam positioned within a container or outer shell which is strapped to the chassis of a vehicle. The outer shell of fiberglass and impact-absorbing foam contained therein serves to protect the cylinders from impact loads.
The known mounting systems typically utilize multiple-component, complete and heavy frames into which cylinders are mounted or rely on foam alone to absorb impact and protect the integrity of the vessels.
What is desired is a lightweight impact-absorption system, which alone or in combination with a mounting system, protects a structure and particularly roof-mounted cylinders both from inertial loading and from direct impact to the cylinders as a result of the impact by absorbing the impact and transferring any residual load away from the cylinders so as to diminish any adverse effects thereon.